Joseph S D Yao jsdy at cospo.osis.gov
Fri Nov 12 16:15:52 UTC 1999

On Thu, Nov 11, 1999 at 12:37:15AM -0500, Michael Dang wrote:
> Thanks for the response.  What the most common mistake that cause problem
> like that?  I just try to narrow down my search for errors here.
> Thanks,
> Mike D.

As I said before,

> Almost certainly you have failed to configure reverse DNS.

I mean you DO NOT HAVE a reverse DNS domain configured.  PERIOD.

I guess it's that time of the week again.

> Subject: Reverse DNS, IP addr -> name via PTR
> Date: Thu, 5 Aug 1999 13:27:38 -0400 (EDT)
> OK, one more time, since there has been a spate of questions about this
> again ...
> If you have a domain and a set of IP addresses, e.g., foo.com and
>, then you will not only want to do forward DNS lookups from
> host names to IP addresses, but also reverse DNS lookups from the host
> names to the IP addresses.  This does NOT happen automatically!
> Instead, you have to construct a separate reverse DNS zone whose name
> is based on the portion of the network that you own.  [I'll mention
> what to do if you don't own the whole network portion, later.]
> There is nothing magic about a reverse DNS zone.  By convention, it is
> based on the "in-addr.arpa" domain.  Its name is constructed on the
> REVERSED IP address of the network - in this case, 3.2.1.in-addr.arpa.
> In all ways, it is a regular domain - its parent domain, e.g., is
> 2.1.in-addr.arpa - we'll get to why that's important in a minute.
> In the named.boot or named.conf, on your master [primary] name server,
> you associate the name of the domain/zone with the name of some file
> that contains the zone information.  In this zone file, you will have,
> as always:
> 	$TTL	nnnnnnn
> 	@	IN  SOA	...
> 		IN  NS	ns.foo.com.
> and then you must put your pointers from the host numbers back to the
> names, e.g.:
> 	1	IN  PTR		router.foo.com.
> 	2	IN  PTR		firewall.foo.com.
> 	3	IN  PTR		host.foo.com.
> 	...
> 	42	IN  PTR		answer.foo.com.
> 	...
> Now, giving your local name server all of this information, it will
> return any reverse-DNS query with the proper information.  E.g.,
> queries of the form:
> 	nslookup    ns.foo.com
> 	nslookup  -type=ptr   ns.foo.com
> 	dig   @ns.foo.com  ptr
> will return the host name as part of the answer.
> Internally to your network, if you have configured your /etc/resolv.conf
> file to point to ns.foo.com's IP address, then you don't even have to
> tell it to ask ns.foo.com.  Queries will automatically go to that name
> server.
> EXTERNALLY to your network, it's a different story.  If you want
> others to also see your reverse DNS [and you usually do], you need to
> get the co-operation of whoever owns your reverse DNS parent domain,
> 2.1.in-addr.arpa.  They must list "3" as a subdomain of their domain,
> with an NS record in their zone file pointing to your name server.
> Then someone asking from the outside about will
> be able to go to the root server, find out who owns 1.in-addr.arpa, and
> from them who owns 2.1.in-addr.arpa, and from them who your name server
> is ... and thence get the name.  Just exactly as in forward DNS lookups.
> NOW, if your network does not break at an octet boundary, you must look
> at RFC 2317, which has a trick to create a subnetwork that includes
> your network name and bitsize, and then have your hosts' IP addresses
> be names off that network.  You can use the trick detailed in RFC 2317
> or one like it.  Believe me, it works.  But you need the co-operation
> of the owner of your parent network.
> If you only have a few IP addresses, or you have IP addresses from
> different networks, you will probably want to just leave forward and
> reverse DNS entries to the owners of those networks.  Again, they have
> to be willing.
> Fair 'nuff?

Joe Yao				jsdy at cospo.osis.gov - Joseph S. D. Yao
COSPO/OSIS Computer Support					EMT-B
This message is not an official statement of COSPO policies.

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